Favoring the Methane Oxychlorination Reaction over EuOCl by Synergistic Effects with Lanthanum

The direct conversion of CH4 into fuels and chemicals produces less waste, requires smaller capital investments, and has improved energy efficiency compared to multistep processes. While the methane oxychlorination (MOC) reaction has been given little attention, it offers the potential to achieve high CH4 conversion levels at high selectivities. In a continuing effort to design commercially interesting MOC catalysts, we have improved the catalyst design of EuOCl by the partial replacement of Eu3+ by La3+. A set of catalytic solid solutions of La3+ and Eu3+ (i.e., LaxEu1–xOCl, where x = 0, 0.25, 0.50, 0.75, and 1) were synthesized and tested in the MOC reaction. The La3+–Eu3+ catalysts exhibit an increased CH3Cl selectivity (i.e., 54–66 vs 41–52%), a lower CH2Cl2 selectivity (i.e., 8–24 vs 18–34%), and a comparable CO selectivity (i.e., 11–28 vs 14–28%) compared to EuOCl under the same reaction conditions and varying HCl concentrations in the feed. The La3+–Eu3+ catalysts possessed a higher CH4 conversion rate than when the individual activities of LaOCl and EuOCl are summed with a similar La3+/Eu3+ ratio (i.e., the linear combination). In the solid solution, La3+ is readily chlorinated and acts as a chlorine buffer that can transfer chlorine to the active Eu3+ phase, thereby enhancing the activity. The improved catalyst design enhances the CH3Cl yield and selectivity and reduces the catalyst cost and the separation cost of the unreacted HCl. These results showcase that, by matching intrinsic material properties, catalyst design can be altered to overcome reaction bottlenecks.


Experimental Definitions and Calculations a. Conversion, Yield, Selectivity and Carbon Balance
The CH 4

b. Determining the Elemental ratio with Vegard's Law
The Origin 2017 multi peak fit tool was used to fit Voigt peaks functions, which in turn were used to determine the (110) X-ray diffraction (XRD) peak positions. This was done for the monometallic catalysts (references) as well as for the bimetallic catalysts and the results are given in Table 1. From the peak position, the interplanar distance d (nm) was calculated according to braggs law, see Eq. S6.
= 2 * sin ( )( . 6) Where λ and θ are the wavelenght of the X-ray source (nm) and the angle of the incident light (˚) to the plane respectively. With the use of the interplanar distance and the Miller indices, the lattice parameters were then calculated. For the tetragonal LnOCl crystal system, Eq. S7 must be used to determine lattice parameters a and c (nm).
For simplicity, either (hk0) can be used to give a or (00l) can be used to give c. The signal splitting is pronounced for the (110) reflection in the region of 29-33° and this reflection was used to calculate the La 3+ :Eu 3+ ratio. The contribution of both elements to each peak was determined via Vegard's law since both diffractions are the same crystal structure. According to Vegard's law, the lattice parameters of a solid solution is approximately the mean of the two lattice parameters, expressed by Eq. S8. 1-3 Where is the average lattice parameter of the alloy, the La lattice parameter and the Eu lattice + parameter. The elemental fraction is expressed by .

Characterization of Spent Catalyst Materials
During the methane oxychlorination (MOC) reaction, catalyst chlorination occurs and a bulk phase transition from LnOCl to LnCl 3 can take place (or at least partly). A dechlorination step in 2:4:1:15 CH 4 :O 2 :N 2 :He was performed at 550 °C to induce a phase transition of the material from the chlorinated phase to LnOCl, thereby removing excess chlorine in the catalyst material and making the sample air-stable. Subsequently, postcharacterization of the catalyst materials with N 2 physisorption, XRD and TEM is performed. However, the physicochemical properties obtained after the post-characterization of the active catalyst material might not be representative of the active catalyst material in the reaction. Nevertheless, the dechlorination step has practical considerations, and without, no post-characterization could be performed. Lanthanide chlorides are hygroscopic in nature and, when exposed to air, form their corresponding hydrates. Upon rehydration, the structure of the catalyst material can be lost as e.g. lanthanum chloride dissolves from moisture in the air. TEM measurements cannot be performed under inert conditions. Furthermore, for XRD and N 2 physisorption, it implies that the catalyst material has to be transported to an inert atmosphere to guarantee the preservation of the physicochemical properties of the active catalyst material. The reactor set-up does not allow us to close the reaction tube and prevent rehydration. Even though the reactor tube can be transferred to a glovebox, we cannot assure that rehydration did not occur. The potential rehydration raises an issue as sorption samples are typically dried at elevated temperatures under vacuum conditions. During this pretreatment, the thermal dehydration can cause hydrolysis of the lanthanide chloride to the lanthanide oxychloride and release HCl. 4,5 The sorption apparatus used in our laboratory is not corrosion resistant and thus the experiment would perform harm to the equipment. XRD can be performed under inert conditions, but its XRD pattern is difficult to analyze as there are many unidentifiable diffractions. Due to these practical considerations, we chose to perform the dechlorination step, as we believe that still some qualitative trends can be deducted from these results. However, we do acknowledge the fact that the physicochemical properties of the catalyst material could be altered during this dechlorination step.

Additional Experimental Data
Figure S1. CH 4 conversion (X CH4 ) and the corresponding selectivity plotted versus the reaction temperature for (A) LaOCl, (B     Table S1. The color of the symbol represents the reaction rate, normalized to the catalyst weight (g catalyst ). (B) The selectivity plotted versus the temperature at which 10% CH 4 conversion is reached for the catalytic systems of Table S1 where stable (chemical, structural and/or catalytic) performance is reported.
(C) Same plot as (B) but normalized to the bed volume (in cm 3 catalyst ) instead of catalyst weight. Table S1. Catalytic systems reported in the academic literature. Temperature, CH 3 Cl selectivity, CO selectivity and reaction rate are given at 10% CH 4 conversion. Subsequently, the reported stability (chemical, structural and/or catalytic) is tabulated. Only the catalytic systems that were reported as exhibiting no stability issues were graphically depicted in Figure S5B.        Figure 7. A gradual decrease in spectral intensity was observed, indicating that EuOCl was chlorinated to EuCl 3 . Still, after 48 h, features corresponding to the luminescence signal of EuOCl were distinguishable, indicating that the catalyst was not entirely chlorinated. For reaction conditions, see Figure 7.